The present invention relates to a shock actuated safety valve which will shut off gas flow in a supply pipe in the event of a major earthquake.
The property damage from a major earthquake can be devastating. Historically, however, much of the worst damage comes after the earthquake is over because of fires that are started during or after the quake. Buildings are especially vulnerable to fire damage at these times because emergency services cannot get through the damaged or rubble-strewn roads to fight the fires. Natural gas supply pipes that are broken or damaged during an earthquake cause a serious danger of fire or explosion. Even a small leak in a gas line can accumulate enough gas in a building to cause a serious explosion if it is ignited.
In the great San Francisco earthquake of Apr. 18, 1906, the actual tremor lasted less than fifteen seconds, but the fires in the aftermath raged out of control for four days, causing more damage than the quake itself. This and similar tragedies point out the importance of having an emergency shutoff valve that will automatically stop the flow of gas into a building when it senses the vibrations of an earthquake of sufficient intensity that it could cause significant structural damage, such as the rupture of gas supply lines.
Recognizing this need, certain regulatory agencies such as the American National Standards Institute (ANSI), and the State Architect of California have established standards of performance for earthquake actuated safety valves. Both the ANSI standard and the California Architectural Code (CAC) require the sensing means of the valve to actuate within 5 seconds when subjected to a horizontal, sinusoidal oscillation having a peak acceleration of 0.3 g (2.94 m/s.sup.2) and a period of 0.4 seconds (a frequency of 2.5 Hz). This corresponds to the type of vibrations experienced in an earthquake with a magnitude of 5.4 to 5.6 on the Richter scale.
The standards also require the valve to be insensitive to vibrations that are not typical of seismic activity. For this reason they require that the sensing means not actuate when subjected for five seconds to horizontal sinusoidal oscillation having:
1) peak acceleration of 0.4 g (3.92 m/s.sup.2) and a period of 0.1 seconds (a frequency of 10 Hz); PA1 2) peak acceleration of 0.08 g (0.78 m/s.sup.2) and a period of 0.4 seconds (a frequency of 2.5 Hz); PA1 3) peak acceleration of 0.08 g (0.78 m/s.sup.2) and a period of 0.1 seconds (a frequency of 10 Hz).
This part of the standard is intended to ensure that the shutoff mechanism is not triggered by harmless vibrations such as a passing vehicle, loud noises, or even minor seismic activity that is not likely to result in structural damage or ruptured gas pipes.
It is the intent and objective of the present invention to meet or exceed these standards and thus provide a safety shutoff valve that automatically stops the flow of gas in the occurrence of a major earthquake yet will not unintentionally stop the gas flow when subjected to other harmless vibrations.
It is a further objective of the invention to provide an earthquake sensing mechanism that is omnidirectional in its sensitivity. It is important that the shutoff valve actuate at the correct intensity of oscillations no matter what direction the vibrations are coming from.
It is also an objective to make a shutoff valve that can be easily reset after it has been activated so that gas flow can be reestablished after the danger has passed and the gas pipes have been checked for leaks.
Yet another objective is that the shutoff valve, once it has been activated, should stay closed even if the structural damage to the building is so severe that the valve is tilted 45.degree. from its vertical position.